1. Field of the Invention
The present invention relates to a honeycomb-type piezoelectric/electrostrictive element. More particularly, the present invention relates to a honeycomb-type piezoelectric/electrostrictive element which exhibits excellent durability, is suitable for weight reduction, and can be produced at low cost.
2. Description of Related Art
A fast-response piezoelectric actuator is used as a fuel injection valve actuator for a diesel engine and a precise positioning actuator. A piezoelectric actuator generally has an extremely large generative force of about 3 kg/mm2, but produces a small displacement of about 0.1 mm or less. Therefore, measures for increasing the displacement of the piezoelectric actuator have been tried.
As an actuator having the increased displacement, at the moment, there is mainly proposed a stacked piezoelectric actuator in which thin piezoelectric sheets provided with an electrode are stacked and unified (see patent documents 1 to 3, for example). An actuator having a vortex structure is also proposed (see patent document 4, for example). Among them, a stacked actuator is generally formed using the following method. First, an unfired tape (sheet) with a thickness of several tens of micrometers is formed using a doctor blade method or the like, and is cut to appropriate dimensions to obtain a green sheet. Next, an electrode paste containing a metal such as Pt or Ag/Pd which can withstand the piezoelectric sintering temperature is printed on one side of the green sheet as an internal electrode metal paste, and several hundreds of green sheets are stacked. After removing the binder, the resulting unfired laminate is fired (sintered) at 1000° C. or more. After cutting the sintered body to the desired dimensions and shape, each internal electrode is connected to an external electrode provided to the side surface of the sintered body to obtain a stacked actuator.
[Patent document 1] JP-A-6-120579
[Patent document 2] JP-A-2002-261339
[Patent document 1] JP-A-2004-297042
[Patent document 4] JP-A-11-112046
In such a stacked actuator, generally, an inactive region (inactive portion) in which the electrode is not formed exists in the peripheral portion of each layer. Since a tensile stress due to driving and deformation of the active portion occurs in the inactive portion, cracking or the like occurs, resulting in deterioration in durability. To solve this problem, a stacked actuator having a configuration in which an electrode is formed over the entire surface is proposed. However, such a stacked actuator requires a complicated step of providing a coating formed of an insulating material almost over the entire width of the edge of the internal electrode exposed on the side surface (see patent document 1). Moreover, the external electrode connecting the electrode between the layers tends to break due to tensile stress caused by the expansion/contraction movement of the actuator. This also results in deterioration in durability. Therefore, a complicated step such as attaching a metal mesh is required (see patent document 2). Since a stacked actuator tends to undergo delamination or the like due to shear stress which occurs between the piezoelectric layer and the electrode layer, it is necessary to operate the stacked actuator while applying an appropriate preload (see patent document 3).
Since a lead-containing piezoelectric material with a specific gravity of about 8 is generally used for a stacked actuator, the mass of the stacked actuator tends to be increased when forming a dense bulk structure. A cylindrical actuator is also proposed in order to ensure the displacement area and the displacement magnitude while suppressing an increase in mass. However, there is a problem that the processing cost increases when the aspect ratio of the element is increased.
A stacked actuator is generally produced by a large number of steps, resulting in an increase in cost. In the stacked actuator production process, the piezoelectric material and the internal electrode are generally fired at the same time. Therefore, it is necessary to use a material containing an expensive noble metal such as Pt or Ag/Pd with a heat resistance of 1000° C. or more for the electrode, resulting in an increase in cost.
A actuator having a vortex structure is proposed in order to improve the durability of the stacked actuator (see patent document 4). In this actuator having a vortex structure, a piezoelectric ceramic layer and a vibrating electrode are wound in a vortex shape to form an approximately cylindrical shape. This vortex structure is advantageous as compared with the stacked structure in terms of durability. However, problems of a large mass and a high production cost have not been improved.